Field of the Invention
The invention relates to semi-finished products made of aluminum alloy manufactured by vertical direct chill semi-continuous casting such as rolling slabs and extrusion billets, more particularly, such semi-finished products, their manufacturing methods and use, designed in particular for aeronautical and aerospace engineering.
Description of Related Art
Aluminum alloy plates and thick sections are used in particular in the aeronautical and aerospace engineering industry. These products are obtained in general by a method including vertical semi-continuous casting of a semi-finished product, rolling slab or extrusion billet, optionally the homogenization, hot working by rolling or extrusion, solution heat treatment and quenching of an aluminum alloy. The most frequently used alloys are alloys of series 2XXX, series 7XXX and certain alloys of series 8XXX containing lithium.
These products must in particular possess certain customary properties in particular in terms of the compromise between static mechanical resistance properties (tensile and compression yield stress, breaking strength) and damage tolerance properties (fracture toughness, resistance to fatigue crack propagation), these properties being in general contradictory.
It is known that damage tolerance properties may be influenced in particular by the presence in thick products of non-metallic inclusions and microporosity.
Micropores appear during slab casting and are then partially or completely filled by the rolling process. It is therefore attempted to eliminate micropores greater than approximately 90 μm which prove particularly detrimental for damage tolerance.
U.S. Pat. No. 5,772,800 describes a method for obtaining plates of thickness greater than 50 mm characterized by a density of micropores of size greater than 80 μm of less than 0.025 micropores per cm2 and a microporosity volume for the slab of less than 0.005%, in which the hot rolling conditions and the reduction ratios are adapted according to the radius of the hot rolling cylinder.
This method requires special tools for hot rolling and in certain cases, depending on the tools available and the thicknesses sought after, it is not possible to attain the transformation conditions that would allow the porosities to be filled efficiently.
It is therefore desirable to obtain, as of the casting stage, a semi-finished product with a low density of large-sized micropores and a low microporosity volume.
It is generally accepted that degassing of the molten metal makes it possible to decrease the quantity of micropores in particular by decreasing the hydrogen content. The hydrogen content in the molten metal is measured, for example, using a probe of the Telegas™ or Alscan™ type, known to those skilled in the art.
Known methods for decreasing the hydrogen content are, for example, treatment in a degassing ladle using a rotor by introducing chlorine and/or argon. The use of ultrasonic emissions to degas the molten metal is also known.
Patent application CH 669.795 describes, for example, the introduction of ultrasonic heads into a conveying trough from a furnace to a foundry so as to obtain degassing.
Patent application US2007/235159 describes apparatus and a method in which an ultrasonic vibration is used to degas the molten metal in the presence of a purge gas such as argon or nitrogen.
International application WO00/65109 describes a device for injecting gas bubbles into a molten metal, in which the diameter of the bubbles can be decreased using ultrasounds.
U.S. Pat. No. 4,546,059 describes a method for continuous casting of light-alloys wherein ultrasound treatment is carried out in a solidification device optionally in an area separated from the solidification front by a porous material. This method results simultaneously in the formation of a uniform sub-dendritic structure, lowering of hydrogen content and reduction of porosity. To position the ultrasound head in the solidification device is not convenient.
The reduction in the hydrogen content proves to be difficult to obtain and depends on external conditions such as ambient moisture. It would therefore be advantageous, for a constant hydrogen content, to be able to limit the presence of large-sized micropores.
Moreover, one of the consequences of later heat treatments of the solidified metal, in particular including homogenization, generally necessary for metallurgical reasons, is to increase the dimension of micropores.
Last, a simplified method, alternative to existing methods which are often difficult to carry out would be advantageous.
There exists a need, for an improved method to obtain unwrought semi-finished aluminum alloy products such as rolling slabs or extrusion billets, in particular made of alloy 2XXX or 7XXX, having a low density of micropores of size greater than approximately 90 μm, in particular in the homogenized state, and for a device to carry out such a method.